Glycopeptides of molecular weight range 7-11,000, unusual in size and structure, have been partially purified from pronase digests of lipid-free human erythrocyte ghosts; we term this fraction "erythroglycan". These substances comprise about 1/3 of the galactose and glucosamine of the ghost, and contain the repeating structure: N-Acetylglycosaminyl-(1 yields 3) galactosyl (1 yields 4): This substance has been overlooked as a major carrier of carbohydrate of the erythrocyte stroma since its structure is almost entirely resistant to periodate-schiff staining of membrane proteins on polyacrylamide gels. The repeating unit is the same as that for keratan-like structures and for the core units of ABO substances, neither of which has been reported as a cell surface glycopeptide. We propose here to characterize the components of this fraction in chemical detail and to identify the proteins to which the chains are attached. Since this molecule may be a carrier of ABO determinants, we intend to examine structures from Bombay, O, A, B, and AB cells. In addition, we plan to examine porcine, ovine, bovine, equine and rodent erythrocytes for similar structures to see whether extended glycopeptides are a general property of erythrocytes. Erythrocyte stroma are delipidated in chloroform-methanol and lyophilized. After pronase digestion, and chromatography on concanavalin A-Sepharose, the unbound fraction is treated with base-borohydride and chromatographed on G-50 Sephadex. The fraction of 8-12,000 molecular weight is pooled, comprising more than 1/3 of the erythrocyte carbohydrate. Methylation analysis is performed using combined gas-chromatography and chemical-ionization mass spectrometry. The erythroglycan is partially degraded with endo-beta-galactosidase from E. freundii and is completely depolymerized to disaccharides with hydrazinolysis-deamination.